High spectral efficient digital modulation over radio channels requires the use of multilevel/multiphase signals. This type of signal is very sensitive to time-varying amplitude and phase distortion associated with land mobile communications caused by Rayleigh fading.
Prior art systems have addressed this problem by transmitting a carrier pilot with the data modulation. This pilot is used at the receiver to compensate for the time varying amplitude and phase distortion mentioned above. Usually the pilot is extracted at the receiver by a bandpass filter. Assuming there is no frequency offset between the transmitter and the receiver, the design of the bandpass pilot extraction filter must take into consideration the following conflicting goals:
First, its bandwidth must be wide enough to pass the spectrum associated with the Rayleigh faded pilot signal.
Second, its bandwidth must be narrow enough to preserve the spectral efficiency of the system, and also to keep the noise associated with the pilot extraction as low as possible.
To improve the sensitivity of the transmission system without increasing its bandwidth, trellis coded modulation (TCM) is used. TCM schemes use redundant nonbinary modulation, in combination with a finite state encoder that governs the selection of modulation signals to generate coded signal sequences. Historically TCM has been used to improve the transmission performance over channels where the main source of errors is additive noise. Recently TCM has been proposed to combat the effect of Rayleigh fading in mobile communications. However, there are problems with this approach, as follows:
In a pilot aided transmission, the phase and amplitude of the Rayleigh fading component could be known if the bandwidth of the pilot extraction filter were sufficiently wide and if there were no noise associated with the pilot extraction. However, this is unrealistic, since the limited bandwidth of the pilot extraction filter makes that during deep fades, when rapid phase variations are more likely to appear, the output of the filter cannot follow the phase at its input. Also, the noise associated with the pilot extraction is high.
Further, at the receiver, a Viterbi decoder with Euclidian metric is used to recover the transmitted information from the trellis coded sequence. This type of decoder is optimum if the noise associated with the samples at the channel output is Gaussian and its samples taken at the symbol rate are uncorrelated. However, neither assumption would apply in the case of land mobile radio receivers.
As a result of the above, the use of a Viterbi decoder with Euclidian metric in a Rayleigh fading environment makes performance of the TCM scheme worse than its uncoded counterpart. The reason for this is the rapid phase variation when the signal undergoes a deep fade. Therefore, there is a need for an improved TCM decoder for the land mobile environment.